1. Field of the Invention
The present invention relates to a liquid crystal device used in a liquid crystal display device, a liquid crystal optical shutter or the like. More particularly the present invention is concerned with a liquid crystal device having achieved an improvement of alignment characteristics of liquid crystal molecules, and a display system in which such a liquid crystal device is used for display.
2. Related Background Art
Clark and Lagerwall have proposed a display device of the type the refractive index anisotropy of ferroelectric liquid crystal molecules is utilized and transmitted light rays are controlled by its combination with a polarizer (see Japanese patent Application Laid-open No. 56-107216 and U.S. Pat. No. 4,367,924). This ferroelectric liquid crystal commonly has a chiral smectic C phase (Sm*C) or H phase (Sm*H) of non-spiral structure in a specific temperature range, and, under such a state, has the properties of assuming either a first optically stable state or a second optically stable state in response to an electric field applied and also maintaining that state when no electric field is applied, in other words, the bistable state (or two stable states). This ferroelectric liquid crystal also has a quick response to changes in electric fields, and is expected to be widely utilizable as a display device of a high-speed and memory type. In particular, in view of its function, it is expected applicable as a display device with a large screen and high resolution.
In order for an optical modulation device making use of such a bistable liquid crystal to exhibit drive characteristics, a liquid crystal disposed between a pair of parallel substrates is required to be in the state of molecular orientation that the conversion between the above two stable states may effectively take place.
In the case of a liquid crystal device in which the double refraction of a liquid crystal is utilized, the transmittance under cross polarization is expressed by: ##EQU1## wherein I.sub.O : incident light intensity.
I: transmitted light intensity. PA0 .theta.: tilt angle PA0 .DELTA.n: refractive index anisotropy PA0 d: film thickness of a liquid crystal layer, PA0 .lambda.: wavelength of incident light.
Thus, the tilt angle .theta. in the above non-spiral structure is present as an angle in the direction of an average molecular axis of liquid crystal molecules twisting oriented in the first and second states of alignment. According to the above expression, the transmittance reaches a maximum when the tilt angle .theta. is at an angle of 22.5 . and hence a tilt angle .theta. in the non-spiral structure that effects the bistable state is required to be as near as possible to 22.5.degree..
Incidentally, as a method of aligning ferroelectric liquid crystal, a liquid crystal molecular layer formed of a plurality of molecules that form a smectic liquid crystal over a large area must be monoaxially aligned along its normal and hence a polyimide film usually having been subjected to rubbing has been widely used. In particular, the method as disclosed, for example, in U.S. Pat. No. 4,561,726 is known as a method of aligning a chiral smectic liquid crystal of non-spiral structure. However, in instances in which hitherto available alignment methods using a polyimide film having been subjected to rubbing are applied to the above bistable ferroelectric liquid crystal of non-spiral structure as reported by Clark and Lagerwall, there are the following problems.
That is to say, experiments made by the present inventors revealed that a tilt angle .theta. (the angle shown in FIG. 3 as described later) in ferroelectric liquid crystal of non-spiral structure obtained by making alignment using the conventional polyimide film having been subjected to rubbing becomes smaller than a tilt angle H (the angle of 1/2 of a vertical angle of a trigonal pyramid shown in FIG. 2, as described later) in a ferroelectric liquid crystal of spiral structure. In particular the tilt angle .theta. in a ferroelectric liquid crystal of non-spiral structure obtained by making alignment using the conventional polyimide film having been subjected to rubbing was approximately 3.degree. to 8.degree. in general, and the transmittance at that time was approximately 3 to 5% at best.
Thus, according to Clark and Lagerwall, the tilt angle in a ferroelectric liquid crystal of non-spiral structure that effects the bistable state should be the same angle as the tilt angle in a ferroelectric liquid crystal of spiral structure. In practice, however, the tilt angle .theta. in the non-spiral structure is smaller than the tilt angle H in the spiral structure. It was also revealed that the reason why the tilt angle .theta. in the non-spiral structure is smaller than the tilt angle H in the spiral structure is ascribable to the twisted orientation of liquid crystal molecules in the non-spiral structure. More specifically, in the ferroelectric liquid crystal of non-spiral structure, liquid crystal molecules are, as shown in FIG. 4, twistingly oriented in a continuous form at a twist angle .delta., which is twisted from an axis 42 of a liquid crystal molecule adjacent& to an upper substrate toward an axis 43 of a liquid crystal molecule adjacent to a lower substrate (i.e., in the direction 44 of twisted orientation), with respect to the normal of each substrate. This is the reason why the tilt angle .theta. in the non-spiral structure becomes smaller than the tilt angle H in the spiral structure.
In FIG. 4, the numeral 41 denotes a monoaxial alignment axis formed on upper and lower substrates obtained by rubbing or oblique vacuum deposition.
In the instance where a chiral smectic liquid crystal is aligned using a conventional polyimide alignment film having been subjected to rubbing, the polyimide alignment film is present as an insulating layer between an electrode and a liquid crystal layer. Hence, when a voltage with one polarity is applied so that the first optically stable state (for example, a state of the display in white) is switched to the second optically stable state (for example, e state of the display in black), a reverse electric field V.sub.rev with the other polarity is produced in a ferroelectric liquid crystal layer after removal of the application of this voltage with one polarity, and this reverse electric field V.sub.rev has often caused an after-image at the time of display. (see, yoshida Akio. "Switching Characteristics of SSFLC", Proceedings for Liquid Crystal Forum, p. 142-143, October, 1987.